I was writing unit tests and needed to cause this function to kick out an error and return FALSE in order to test a specific execution path. If anyone else needs to force a failure, the following inputs will work:
<?php
parse_url("http:///example.com");
parse_url("http://:80");
parse_url("http://user@:80");
?>